Applied daily for 7 days to soften hyperkeratosis and facilitate its removal

12% salicylic acid in collodion

Macerating agent; painted over callosity and left in situ for 7 days, to assist removal of heavy callosity

20-40% salicylic acid plaster

Applied topically and left in situ for 1-2 days to aid removal of corns

40-70% salicylic acid ointment

Applied topically in a cavitied pad and left in situ for 7 days for verruca treatment; the lesion should be masked

Calmurid cream (10% urea)

To hydrate anhidrotic skin; applied daily to treat dyskeratosis

40% urea cream

A strongly keratolytic agent, applied under an occlusive dressing and left in situ for 7 days, to soften, macerate and aid the reduction and removal of hypertrophied nails in patients who are unsuitable for nail avulsion

Monochloroacetic acid

A deeply penetrating caustic that is painful in use Applied, retained in situ and reviewed within 3-7 days, for the treatment of verrucae:1. as a saturated solution to the lesion2. as a tiny crystal strapped over a masked lesion3. as a tiny crystal embedded in 40-70% salicylic acid retained by a cavitied pad

Trichloroacetic acid

A self-limiting caustic with a superficial actionApplied directly to the verruca, after initial scalpel debridement of the lesionMay be used in conjunction with 75-95% silver nitrate (see below) as a diagnostic or a final treatment of verrucae

75-95% Silver nitrate

A self-limiting caustic with a superficial action, causing a dark brown discoloration of the skin to which it is applied; it is used for the treatment of shallow or mosaic verrucae.Note: Some patients show an idiosyncratic local sensitivity or inflammatory reaction to applied silver nitrate1. Applied directly to the lesion, after overlying callosity has been debrided off2. Applied directly to the lesion in alternating layers with trichloroacetic acid3. As a diagnostic aid to identify verrucous tissue; viral-infected skin cells show up as bright white dots within a few moments of the application of the layers of silver nitrate and trichloroacetic acid

Potassium hydroxide (KOH)

A powerful keratolytic caustic with a great affinity for water that penetrates deeply dissolving precipitated protein; used to destroy soft tissues1. Overlying hyperkeratosis is debrided off the verruca and the foot is immersed in water for 5 minutes to hydrate the skin, then dried; a KOH pellet is rubbed into the lesion; the foot is reimmersed in water and the jelly-like material formed by the KOH on the lesion surface is debrided off. The process is repeated once or twice until the lesion appears to have gone, then glacial acetic acid is applied to neutralize the KOH2. A similar protocol may be used to ablate nail matrix after removal of the overlying section of nail plate3. KOH 5% liquid applied to heavy callosity and left in situ for 5 minutes softens heavy callosity to ease its removal

Pyrogallol (pyrogallic acid)

A powerful analgesic, non-self-limiting, caustic reducing agent. It may be used in the treatment of recalcitrant verrucae or neurovascular corns in areas of skin overlying a healthy layer of fibrofatty padding. It should only be used with great caution as its action continues after application has ceased and can lead to severe tissue breakdown that is slow to heal. It is incompatible with alkalis, iron salts, oxidizing agents and ammonium salts1. 20% pyrogallol ointment for the treatment of neurovascular corns2. 40% pyrogallol ointment for the treatment of VP3. WP ointment (20% pyrogallol, 20% wheat germ oil) for the treatment of tough, fibrous, hyperkeratotic plantar lesions

Phenol

An analgesic, corrosive caustic. It is used as an 80% solution (liquefied phenol) to destroy soft-tissue lesions such as VP, or nail matrices (three applications, each of 1 minute duration). Its action is quenched by dilution with IMS or isopropyl alcohol. Healing is delayed for several weeks after its application

Glacial acetic acid

A weakly acidic mild caustic that is crystalline at 14°C1. As a paint to hard or vascular corns, or VP (return period 14-21 days)2. As a paint to VP, alternating with silver nitrate 75% (in a similar manner to trichloroacetic acid)3. To neutralize KOH (see above: KOH, point 1)

Nitric acid

A powerful analgesic oxidizing caustic agent with a superficial action that offers a 'one-off' VP treatment1. Applied to VP with a glass rod and left in situ for 5 minutes, followed by phenol solution 10%; the skin stains bright yellow2. The lesion is saturated with phenol solution 5% for 5 minutes, then with nitric acid for 20-30 seconds, then once again with phenol solution 5%

Strong iodine solution (iodine solution 10%; iodine fortis)

A strong astringent and vesicant agent. It is incompatible with many topical medicaments, and can cause sensitivity reactions in some patients1. to shrink nail tufts2. to shrink hypergranulation tissue

Formaldehyde

A strongly astringent and antiseptic agent used in the treatment of VPs (the skin surrounding the lesion should be protected with petroleum jelly; sensitivity is likely)1. 10% formaldehyde in collodion, painted on daily2. 36% formaldehyde solution, painted on daily

Thermal caustics

Cryosurgery

The topical application of liquid nitrogen (at -196°C) or nitrous oxide (at -88.5°C) to destroy small soft-tissue lesions; the cell cytoplasm must be reduced to and maintained at -24°C or lower for at least 1 minute, and repeated for two further freezing episodes between which the area has been allowed to thaw. Cryosurgery is more effective when any overlying hyperkeratosis is removed before freezing

Hyfrecation

Tissue destruction by initial fulguration (outlining and superficial charring) of the lesion by the application of high-frequency electrical energy), then electrodesiccation (electrocautery) of the lesion by the release of electrical energy whilst the probe is inserted into the lesion

Electrosurgery

Tissue removal using high-frequency energy waves to incise through tissue

Note: Please also refer to the text entries for each listed agent.

IMS, industrial methylated spirit; VP, verrucae pedis.

caustic

The concentration of light in the caustic surface of a bundle of converging light rays which represents the focal image in an optical system uncorrected for spherical aberration. It appears as a hollow luminous cusp with its apex at the paraxial focus.

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